Killer ants could take a bite out of cane toad problem

Eat them alive, or at least get predatory ants to do it for us – that’s the latest proposal for dealing with the invasion of tropical Australia by toxic cane toads.

And in an unrelated move, Townsville in North Queensland called on its citizens to take part in a mass capture program – the Toad Day Out on Sunday – leading to the humane destruction of 362 kilograms (798 pounds) of toads, which is around 3600 individuals, according to Andrew Hannay of Townsville council.

Cane toad (Bufo marinus) young are more susceptible to being eaten by Australian meat ants (Iridomyrmex purpureus) than the young of native frogs, raising the possibility that meat ants might make a safe biocontrol agent, according to a team led by Rick Shine of the University of Sydney.

“The next step is to work out whether we can manipulate habitats – perhaps by using bait to bring toads and ants together more frequently – to increase the kill rate,” says Shine.

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Toxic wave

According to ecologist Ross Alford of James Cook University in Townsville, the proposal is interesting, but “you would try it cautiously”.

“Ants are generalist predators, so if you increased their numbers, you may see an effect on native animals apart from frogs,” he says. “As we grow to understand [cane toad] biology more fully, other possible control mechanisms will emerge.”

Previous suggestions on how to control them have included importing exotic diseases, raising concerns that the diseases themselves could end up causing as much havoc as the cane toad. Being natives to the continent already, meat ants are likely to be safer, says Shine.

Toxin no defence

Shine’s proposal is based on lab studies confirming field observations that meat ants and cane toadlets are active in the day – a time when most native froglets are inactive.

When housed together, cane toadlets also made physical contact with ants significantly more often than native frogs. And when the ants attacked, which they did at the same rate whatever the species, the cane toads moved away more slowly, allowing the ants to hold on for significantly longer&colon; for more than 5 seconds compared to 1 second for native frogs.

Tests of sprint speed and endurance of 40 young cane toads and 40 young of each of seven native frogs, found the toads to be significantly slower. They also had the lowest endurance of all species – refusing to hop when tapped after little more than one minute – bar the ornate burrowing frog (Opisthodon ornatus). However, ornate burrowing froglets were by far the most vigilant of all the species, moving away when ants approached to within 10 centimetres, says Shine.

Not surprisingly, toadlets were also worse than native frogs at escaping a simulated ant attack – being tapped five times on their bottoms with a pen.

“They’ve evolved in a world without big predatory ants, so instead of hopping away like a sensible frog, they just freeze. That makes sense if [the predator] is affected by your toxin, but the ants aren’t,” says Shine.

Prolific breeders

Cane toads at Australia’s invasion front have evolved longer legs and are faster. However, they are long way from being able to escape meat ants by developing the tactics of native frogs, which can often jump several times their body length, says Shine. “By the second hop, they are in a different world. Cane toads have short stubby legs, and walk rather than hop.”

According to Shine, the Townsville initiative will likely reduce numbers, but only in the short term. “There’s been a tremendous community effort,” he says, but “with females laying up to 30,000 eggs in a single clutch, it’s not a long-term strategy.”